1. Field of the Invention
The invention relates to a forming paste, for example in the form of a sprayable putty, wherein the composition substantially consists of a binder and an electrically conductive component.
2. Description of the Related Art
Such forming pastes are well-known. U.S. Pat. No. 4,279,783 relates to electrically conductive silicone elastomers wherein the addition of a small amount of carbon fibres enhances the electrical conductivity. The problem of these prior art forming pastes is the (relatively) high electrical resistance value thereof. Other conductive fillers can be added to the forming paste so as to reduce said resistance value. This is not desirable, however, as it causes the viscosity to increase to such an extent that such a forming paste is no longer suitably workable for a great many purposes.
It is an object of the invention to provide a forming paste, preferably in sprayable form, which forming paste is suitable for filling a space within a steel housing containing cables for the purpose of electrically connecting said cables to the housing as protection against electromagnetic overtension and/or interference.
Thus the forming paste will form a bushing device to protect against electromagnetic overtension and/or interference.
Another object of the invention is to provide a fire-resistant, electrically conductive paste which is chemically resistant and which can easily be used, in particular as a bushing device for any type of cable diameter and a non-predetermined number of cables.
To this end the invention provides a forming paste which is composed so that a signal attenuation of at least xe2x88x9230 dB or higher takes place in the range of 30 MHz-1 Ghz in the electromagnetic interference measurement according to the EMC IEC 11.9.1 standard.
In order to accomplish the above objectives, the invention furthermore provides a forming paste wherein the binder is selected from the group of elastomers, preferably silicones, and wherein the electrically conductive material is carbon-based, preferably expandable graphite, since this form of graphite also exhibits fire-resistance. Said binder and said form of carbon are advantageous with a view to achieving a low resistance value and a good viscosity. More preferably, oil and/or other agents, such as anti-static agents and viscosity-reducing agents, are added, as a result of which in particular a good workability is obtained.
Surprisingly, the invention provides a forming paste which has a suitable viscosity, as a result of which it is capable of being sprayed, thus enabling a wide range of applications, and which has a surprisingly low resistance value.
The forming paste according to one preferred embodiment of the invention will be marketed under the name of CONDUCTON(copyright), which is a brand name of Beele Engineering B.V. Said CONDUCTON(copyright) forming paste has additional properties:
specific weight 1.4xc2x10.03 g/cm3 
tensile strength 0.8 Mpa
elongation at break 40%
hardness 30 Shore A
elasticity approx. 25%
and in addition it adheres well to metal surfaces, such as steel, zinc and copper. The resistance of CONDUCTON(copyright) forming paste is less than 100 ohms and in addition it is water repellent.
One preferred use is the use of the forming paste as a bushing device for at least one cable to provide protection against electromagnetic overtension and or interference.
A bushing device in the form of an non-deformable plug provided with holes is disclosed in the present applicant""s Dutch patent application N1-1008522. Said plugs are limited in use both by the number of bores and by the diameter thereof, which bores are formed in advance, so that the plug cannot be adapted any more once it has been manufactured. The object of the invention is to provide a bushing device which exhibits a high degree of adaptability. This object is accomplished by using the forming paste according to the invention as a bushing device.
Electrical installations and machines, wherein generally highly sensitive components are used for control and registration purposes, are easily subject to interference by various external electromagnetic fields and pulses. Said external electromagnetic fields and pulses cause a great deal of malfunctions, which are undesirable, and they are usually enclosed via steel constructions, which function as a Faraday cage. Electrical cables, which are led in and out of such a shielded space, can conduct undesirable electromagnetic fields and pulses into the electronic equipment as yet, however. Said conduction will generally take place over an electrically conductive shield (braiding) of the respective cables that are led through. Because of this, the cable is partially stripped of its insulation sheath and is earthed via the forming paste according to the invention and its housing. The prior art bushing plug is pre-formed, however, so that the diameter of the cables and of the bushing channel as well as the number of cables need to be determined in advance.
The use of the present forming paste as a bushing device provides an adaptable mass, so that neither the dimension of a bushing channel nor the number of cables need to be known in advance.
Preferred embodiments of the composition of a forming paste according to the invention are shown below on a weight basis.
Possible trade names of the products that are used are:
Experiment
The electrical conductivity of a forming paste according to the invention was measured by experiment. The electrical resistance was less than 1,000 ohms at all times, and preferably less than 100 ohms. The resistance values are determined via ISO 2878 standard methods in all cases.
A sample plate dimensioned 100xc3x97100xc3x9710 mm was used for determining the resistance value. Square electrodes dimensioned 25xc3x9725 mm, which were spaced 50 mm apart, were applied to the surface of the test piece by means of Aquadag (a colloidal suspension of carbon in water). Resistance values of 80-90 ohms and even lower were recorded.
The invention, in one special embodiment thereof, relates to the use of the electrically conductive forming paste in a multi-cable bushing, which use will be understood more clearly when studied in conjunction with the appended drawings.